The nature of normal grain growth in the presence of a second phase deserves special consideration. The moving boundaries will be attached to the particles exert a pulling force on the boundary restricting its motion. Therefore if the boundary intersects the particle surface at 90 the particle will feel a pull of (2 r cos )*sin . This will be counterbalanced by an equal and opposite force acting on the boundary. As the boundary moves over the particle surface changes and the drag reaches a maximum value, this happens when becomes______

• A. 90
• B. 45
• C. 60
• D. 30